1. Field of the Invention
The present invention relates to an improved pneumatically controlled spring brake of the type utilized in heavy duty vehicles, such as commercial truck tractors and trailers.
2. Description of the Prior Art
Pneumatic spring brakes are widely utilized in the trucking industry. A typical conventional pneumatic spring brake has both a service brake portion for slowing and stopping a truck or trailer moving under normal conditions, and an emergency or parking brake portion. The service brake portion is operated by means of a foot valve located in the vehicle cab. The service brake portion of the pneumatic spring brake applies the brakes to the wheels of a truck or trailer on the highway using pneumatic pressure to act against a diaphragm located within the service brake housing. Air under pressure is utilized to activate the service brake portion.
In a typical pneumatic spring brake air is maintained in a pneumatic reservoir or tank carried on board the vehicle at a pressure on the order of about 100 psi. The reservoir provides 3,000 pounds of thrust against the 30 square inch effective surface area of the service brake diaphragm. Pneumatic pressure is maintained by an on board air compressor. If a failure develops in the pneumatic air supply system, however, the service brake will not function. In this event the emergency brake will thereupon operate automatically.
In a double diaphragm spring brake the emergency brake, which also serves as the parking brake, is mounted in tandem with the service brake. The actuation force of the emergency brake is applied to the brake mechanism on the vehicle wheel through the service brake push rod. Application of the emergency brake is entirely mechanical. The actuating force for the emergency brake is applied by a heavy coil power spring located in a spring chamber of the pneumatic spring brake. This heavy power spring is normally maintained in check by air pressure acting on the opposite side of the spring brake diaphragm in the brake housing while the vehicle is in operation.
The power spring within the spring brake portion of the device applies the brake on a wheel to which it is connected when there is a loss of air pressure to the spring chamber of the emergency brake. Such a loss of pressure can occur either through a break or failure in the air line that applies pressure to the spring brake chamber, or by intentional exhaustion of air from the parking brake chamber, as when the truck or trailer is parked. In this situation the spring brake serves as a parking brake.
When compressed within the separable brake housing portions which historically have been joined together by semicircular clamping rings, the compressed power spring contains a tremendous amount of potential energy. The power spring is quite heavy and typically weighs three pounds or more. The power spring is compressed to a linear length of less than three inches from an original uncompressed length in an extended condition of from nine to twelve inches. Consequently, the compressed spring in the pneumatic spring brake housing can be extremely dangerous, especially to persons who are unaware of the danger or are unfamiliar with the structure and manner of interaction of the component parts of the spring brake. Even when pressure in the emergency brake housing is exhausted, the spring is held in a compressed condition so as to exert a force on the housing of from 750 pounds to 1750 pounds, if an unskilled person or a person unfamiliar with pneumatic spying brakes attempts to take the spring brake apart for repair, the power spring will literally explode from the housing and can cause profound bodily injury, and even death.
In some conventional spring brakes, the clamping ring which joins the separable housing portions of the spring brake is constructed of two generally semicircular clamping ring elements having radially outwardly turned attachment ears at their ends. These semicircular elements are bolted together by bolts extending through openings in the ears. Each of the housing portions is formed with a radially outwardly directed lip or rim. These outwardly directed lips or rims are pressed toward each other throughout their perimeters.
When nuts are tightened on the bolts joining the clamping ring ears the distance of separation between the juxtaposed attachment ears is reduced as the bolt assemblies draw the clamp ring elements toward each other. This causes the clamping ring elements to firmly grip and secure the lips of the separable emergency brake housing portions of the spring brake together. The lips or rims of the two adjacent housing portions are entrapped and held together by the clamping ring. The clamping ring encircles the housing portions at their mutual interface. However, any person with only the most rudimentary of workshop tools can unthread the nuts from the clamping ring bolts and thereby inadvertently release the tremendous power of the coil spring.
One approach toward reducing the potential for injury when a spring brake is disassembled is to "cage" the spring prior to disassembly so that the spring exerts no pressure against the spring brake diaphragm. Caging is performed by inserting a special bolt through an access hole in the end of the spring housing. The bolt is inserted concentrically through the coils of the spring, and into an opening in a pressure plate. The bolt is equipped with a transverse cross pin at the end which is inserted through the pressure plate. The pressure plate includes a slot to admit the cross pin therethrough.
Once the cross pin clears the pressure plate the bolt is turned so that the cross pin is brought into contact with a recess in the underside of the pressure plate. A nut is threadably engaged on the threaded end of the bolt which protrudes from the access hole in the housing. The nut is tightened down to bear against the housing adjacent the access hole. Tightening of the nut at the exposed end of the bolt draws the pressure plate toward the end of the housing in which the access hole is formed, thereby retracting the spring pressure plate and the coiled spring away from the diaphragm.
Caging of the spring can be quite dangerous to persons who are uninformed as to the construction of a spring brake however, particularly if the pressure plate has corroded or the spring has broken. Furthermore, it is not uncommon for springs to break, due to fatigue, especially when a plug has fallen from the access hole and corrosion has accelerated fatigue. Consequently, the disassembly of a spring brake entails considerable risk of bodily injury and damage.
Another approach toward reducing the potential for injury is to construct the emergency brake portion of the pneumatic spring brake as a permanently sealed unit. Unlike the service brake portion, access to the emergency brake portion is desirable only very infrequently. While the service brake diaphragm is worked back and forth many thousands of times, and will wear our and must be replaced, the emergency brake portion is used far less often. Therefore, by permanently sealing the emergency brake portion with the heavy spring mounted therein, the danger of injury by disassembly of that portion of the spring brake by unskilled personnel or persons lacking knowledge of the construction and dangers of such disassembly is avoided.
One way of permanently sealing the emergency brake portion of the spring brake is to fabricate the emergency brake head with a skirt or flange that can be wrapped over a corresponding mating flange or lip of the portion of an adapter forming the other part of the emergency brake housing once all the components of the emergency brake have been installed within the emergency brake chamber. The emergency brake is constructed of a pair of concave shell-like housing portions which fit together. In the trade the emergency brake head portion is often referred to as the spring chamber and forms one end of the emergency brake cavity. The adapter forms the other end of the emergency brake cavity as well as a portion of the service brake cavity.
The flange or skirt of the emergency brake head may be spun or forced over and wrapped underneath the facing lip of the adapter using a press or by other means so that the two lips or flanges of the emergency brake head and the adapter are permanently crimped together. The outer lip of the emergency brake head is wrapped over and captures the inner lip of the adapter. The periphery of the rubber, disk-shaped emergency brake diaphragm is entrapped between the crimped lips or flanges of the emergency brake head and the adapter. The crimping operation permanently secures the periphery of the emergency brake diaphragm between the emergency brake head and the adapter and forms a pneumatic seal that pneumatically isolates the emergency brake chamber containing the emergency brake spring from the opposite side of the emergency brake cavity formed by the adapter.
By locking the portions of the emergency brake housing together in a permanent fashion such as this, the heavy power spring is permanently encapsulated within the emergency brake chamber of the spring brake housing. Thus, the emergency brake portion of the spring brake cannot be disassembled so that the risk of bodily injury which might otherwise result from tampering is greatly alleviated. One such spring brake of this type is sold by Overland Brakes, Inc., located in Nampa, Id. as the "Black Max" spring brake unit. The construction of this spring brake is described in U.S. Pat. No. 4,850,263.
Another approach which has been utilized to permanently seal the emergency brake housing portion of a vehicle spring brake is to weld the lips of the mating emergency brake housing shell portions together throughout their circumferences. Such a system is described in U.S. Pat. No. 5,285,716. However, this technique has disadvantages in that the welding process is quite time consuming because a welding bead must be formed throughout at least a major portion of the circumference of the spring brake housing. Furthermore, due to the prolonged exposure to heat that is required to create the lengthy weld, there is a considerable likelihood of damage to the rubber emergency brake diaphragm that is gripped between the lips of the shells. Thus, this technique is relatively slow, costly, and can easily result in a permanently sealed, but defective emergency brake unit.
Still another approach to creating a sealed brake unit is to provide an encircling band of U-shaped cross section that is initially split at one or more locations. The band is spread apart and moved longitudinally along the length of the spring brake housing sections until it resides in radial alignment with the projecting lips of the spring brake chamber head and the adapter head. The band is then closed so that the ends of the band reside in abutment. The ends of the band are then fusion welded together so that the band forms an inextendable hoop that encircles and grips the radially projecting lips of the housing components.
One difficulty with all tamper-proof spring brakes that have heretofore been devised is that they are not only tamper proof but they cannot be repaired. Indeed, if the spring within the unit should break or become misaligned or should the spring brake diaphragm become ruptured, the entire unit must be discarded as there is no practical way of separating the housing components without destroying the structural integrity of the spring brake.